The present invention concerns a scratchproof and transparent watch crystal, and in particular a watch crystal having a low cost price independently of the complexity of its shape, for example flat or spherical, while having a very high hardness, of the order of 10,000 Vickers.
The present invention also concerns a watch case fitted with such a crystal.
Watch crystals for the protection of dials and hands or suchlike are most often made either of a synthetic material or a mineral glass because of their relatively low cost price. The respective hardness of these categories of materials of course determines the scratch resistance of the crystals. In order to clarify, a watch crystal made of synthetic materials such as Plexiglas has a Vickers hardness of approximately 100 and natural or mineral glass has a Vickers hardness of approximately 900. Experience has shown that crystals made in these two categories of materials have poor resistance to scratches by certain very hard agents such as the silica contained in dust, marble or even the sand constantly present in our environment, even if mineral glass resists such attacks better than synthetic materials. There therefore results a relatively rapid alteration in the aesthetic appearance of these types of watch crystals or back covers by scratches.
In order to avoid these drawbacks, synthetic sapphire or corundum have been used for manufacturing watch crystals. These crystals are disclosed for example respectively in Swiss Patent No 632 891 and in French Patent No 1 238 069. These crystals resist attacks from external agents very well but have however the major drawback of being long, complex and laborious to manufacture and having a very high cost price, which considerably limit their use on a very large scale. By way of illustration, the electric energy requirements alone represent 80% of the manufacturing cost of the sapphire "pears" (cylindrical rods) from which are cut the plates which, after numerous subsequent machining operations, will form these watch crystals. Moreover, an average sized factory for manufacturing sapphire "pears" consumes annually as much electricity as a town of approximately 50,000 inhabitants.
It is thus easily understood, given current increasing energy saving concerns, the necessity of finding an alternative, and in particular a more economical, solution to the use of synthetic sapphire for making watch crystals having high scratch resistance and intended to be used in a wide range of watches.
Moreover, the production of these crystals mainly during the machining stage of the finished product from these "pears" results in significant raw material wastage.